1. Field of the Invention
The present invention relates to a radio communication system for effecting communication among a plurality of radio stations, a radio communication apparatus and a radio control method, and a computer program. Particularly, the present invention relates to a radio communication system in which a network is configured under the control of a particular control station, a radio communication apparatus and a radio communication method, and a computer program.
More particularly, the present invention relates to a radio communication system in which a plurality of wireless networks coexist, a radio communication apparatus and a radio communication method for controlling a communication operation in each wireless network in a communication environment wherein a plurality of wireless networks contends, and a computer program and, particularly to, a radio communication system, a radio communication apparatus and a radio communication method, and a computer program that are run, obviating interference among contending networks.
2. Description of the Related Art
A plurality of computers may be connected to constitute a local area network (LAN) to share information, such as files and data, and peripheral equipment, such as a printer, and to effect information exchange, including transfer of electronic mail and data contents.
Hitherto, LANs have generally been formed using optical fibers, coaxial cables, twist pair cables or other types of cables. This automatically requires line laying construction work that involves complicated cable routing, making it difficult for achieving easy configuration of networks. Furthermore, after configuring a LAN, equipment movable distances are inconveniently limited by cable lengths of the equipment. As a solution to this shortcoming, attention has been focused on a wireless LAN as a system for releasing users from the LAN wiring based on a conventional cable connection method. The wireless LAN makes it possible to obviate the need for most cables in a working area, such as an office, allowing communication terminals, such as personal computers (PCs) to be moved relatively easily.
With the recent trend toward increasing speed and lowering prices of wireless LANs, the demand for wireless LANs is markedly increasing. In particular, the introduction of a personal area network (PAN) is recently being discussed in order to effect information communication by configuring a small-scale wireless network for a plurality of electronic equipment installed around users. For instance, different radio communication systems are specified by using frequency bands not requiring permits by supervisory authorities, such as a 2.4 GHz band or a 5 GHz band.
According to, for example, IEEE802.15.3, activities for standardizing high-speed radio personal area networks exceeding 20 Mbps are being developed. According to the section, standardization based on a PHY layer mainly using 2.4 GHz-band signals is being promoted.
In this type of wireless personal network, one radio communication apparatus acts as a control station known as a “coordinator.” A personal area network is formed within 10 meters, centering around the coordinator. The coordinator transmits a beacon signal at a predetermined interval, and the beacon interval is defined as a transmission frame period. A time slot used by each radio communication apparatus is allocated for each transmission frame period.
For allocating time slots, a method known as, for example, “guaranteed time slot” (hereinafter referred to as “GTS”) is used. This is a communication method whereby transmission bands are dynamically allocated while guaranteeing a predetermined transmission capacity at the same time.
For example, a contention access period (hereinafter referred to as “CAP”) and a contention free period (hereinafter referred to as “CFP”) are prepared for a MAC layer specified by IEEE802.15.3. To effect asynchronous communication, a CAP is used to exchange short data or command information. To effect stream communication, dynamic time slot allocation based on a GTS is carried out in a CFP so as to perform channel time reservation transmission.
The MAC layer standardized by IEEE802.15.3 is specified such that it conforms to the standard specifications of other PHY layers in addition to a PHY layer using 2.4 GHz band signals. Moreover, standardization efforts are being started to use other PHY layers in addition to PHY layers using 2.4 GHz band signals for the PHY layer standardized by IEEE802.15.3.
Also recently, wireless LAN systems to which a spread spectrum diffusion (SS) method has been applied are being put in practical use. Furthermore, an ultra-wide band (UWB) transmission method has been proposed for a PAN or similar applications.
According to direct spread (DS) method, which is a kind of the SS method, a transmitting end multiplies an information signal by a random coding sequence called a “pseudo noise” (PN) code so as to spread an occupied band and transmits it, whereas a receiving end multiplies the received spread information signal by a PN code so as to effect reverse diffusion, thereby reproducing the information signal.
In the UWB, an impulse signal sequence of an extremely short period of about a few hundred picoseconds is used to produce an information signal to transfer the signal sequence. The occupied band width is equivalent to a band of a GHz order such that the value obtained by dividing the occupied band width by its central frequency (e.g., 1 GHz to 10 GHz) is approximately 1. The occupied band is an ultra-wide band, as compared with a band width normally used for a wireless LAN that employs the “W-CDMA” or cdma2000 method, the SS method or the orthogonal frequency division multiplexing (OFDM) method.
In a recent communication environment wherein information equipment typically represented by PCs has been disseminated and a number of equipment exists in an office in a mixed fashion, all the equipment being connected through wireless networks, a case may be observed where a small working environment is crowded with two or more wireless networks and a plurality of wireless networks coexists in the same frequency band. The “same frequency band” in this case includes the UWB radio communication method in which data is diffused over an extremely broad frequency band to transfer the data.
Especially in the case of a UWB radio communication network, it is highly probable that neighbor radio communication networks contend, since data is diffused over an extremely broad band to transfer data.
Meanwhile, an impulse signal sequence used in a UWB radio communication system does not have any particular frequency carriers, making it difficult to effect carrier sensing. Hence, if the UWB radio communication system is applied to the PHY layer in IEEE802.15.3, therefore, the absence of any particular carrier signals makes it impossible to conduct access control by using the carrier sensing standardized in the foregoing section. Hence, there is no other alternative but to depend on access control based on time division multiplexing.
In case of a small-scale wireless network system, such as a PAN, the existence of each network or a base station is not necessarily secured. Hence, it is required to solve the problem of contention among networks and to accomplish dynamic allocation of bands or resources when a new network is added in the same space or a network is brought in from another place.
According to the specifications of the PHY layer utilizing 2.4 GHz-band signals standardized by IEEE802.15.3 described above, a plurality of other radio communication systems exist in the same frequency band; therefore, the possibility of coexistence with such systems must be considered. According to the standardized network construction, the use of neighbor piconets is conceivable, in which control stations (hereinafter referred to as “PNCs”) of networks operate the piconets, avoiding interference with each other. More specifically, specifications have been prepared on an assumption that an additional piconet is formed in the same space while an existing control station is transmitting a beacon signal.
The method for operating neighbor piconets standardized in IEEE802.15.3 specifies the use of a pseudo-static GTS to achieve coexistence of its own piconet and an associated piconet.
Thus, according to a conventional method, to create neighbor piconets, a plurality of piconets can be operated on the same frequency by disposing their beacons in ranges that are apart from each other.
In a PAN, however, moving a piconet automatically involves creation of another neighbor piconet, making it impossible to avoid overlapping beacon positions between neighbor piconets.
Furthermore, if neighbor piconets happen to share the same transmission period and transmission timing of a beacon signal, then radio communication apparatuses or devices existing in the two piconets cannot detect beacons from a control station, utterly disabling the operation of the piconets. The control station of course does not receive any signals while it is transmitting a beacon signal, so that it cannot detect by itself that its own beacon signal is colliding with another beacon signal.
In addition, according to the specifications in IEEE802.15.3, a device that can no longer receive a beacon signal will not be capable of identifying a frame structure described in the beacon signal, so that it can no longer transmit its information. Thus, there is no means for a device other than the one acting as the control station to send information to the control station.
The method for operating neighbor piconets according to the specifications in IEEE802.15.3 uses a pseudo-static GTS; hence, the allocation of a GTS once set may be changed by a command issued by a control station of a parent piconet.